1. Field of the Invention
The present invention generally relates to a multicolor image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction machine including at least two of those functions, a positional deviation correction method therefor, and a recording medium storing a positional deviation correction program.
2. Discussion of the Background Art
In general, an electrophotographic image forming apparatus, such as a copier, a printer, a facsimile machine, a multifunction machine including at least two of those functions, etc., includes an optical writing unit or exposure unit that writes image information optically on a surface of an image carrier such as a photoreceptor using an optical beam such as laser light.
As such an electrophotographic image forming apparatus, a tandem type multicolor image forming apparatus including multiple image carriers is widely used. In the tandem type multicolor image forming apparatus, different single-color images are formed on the multiple image carriers, respectively. Then, the single-color images are transferred from the image carriers and superimposed one on another on a sheet of recording media, such as a transfer sheet, that is transported by a transport member such as a transport belt in a direct transfer method, forming a multicolor image.
By contrast, in an intermediate transfer method, the single-color images are primarily superimposed one on another on an intermediate transport member as a multicolor image and then the multicolor image is transferred onto the sheet. In this case, the intermediate transport member serves as the transport member for transporting the multicolor image as well.
When multiple single-color images are thus superimposed one on another, relative positions thereof on the sheet can deviate. That is, the different color images may not be properly aligned, that is, may not perfectly coincide, in the multicolor image, a phenomenon that is hereinafter referred to as color deviation.
Therefore, positioning of the images is important to avoid color deviation, and accordingly it is necessary to adjust positions, distances traveled, and/or velocities of movable elements such as the image carrier, the transport belt, and the like.
In order to adjust the position of the transport belt or the intermediate transfer member, in a known image forming apparatus a positioning mark is provided on the transport member, and positional deviation thereof is corrected based on results obtained by detecting the positioning mark.
Moreover, in such an image forming apparatus, start and end of optical writing, that is, exposure timing, should be controlled. In particular, in the tandem image forming apparatus, if start positions of the respective optical beams on the multiple image carriers are mismatched, relative positions of the multiple single-color images will be misaligned, causing color deviation.
The start and end of optical writing can be detected by first and second optical beam detectors respectively disposed at two different positions on a main scanning line, and measuring time periods for the optical beam to travel between these optical beam detectors by counting a predetermined or given clock signal. Then, the counted clock number is compared with a predetermined reference clock number to calculate an amount by which the end of the optical writing is to be adjusted, and thus magnification of the image that is optically written on the image carrier can be adjusted.
In order to control the exposure timing, a known optical writing unit includes a light source, a deflector that deflects and scans a laser beam emitted from the light source in the main scanning direction, an imaging unit that focuses the optical beam on the surface of the image carrier, and multiple laser beam detectors arranged in the main scanning direction that detect a position of the laser beam. Each laser beam detector includes multiple light-receiving surfaces, and at least two of the multiple light-receiving surfaces are adjacent to each other at a given angle.
Another known optical writing unit includes a deflector that deflects an optical beam that is modulated according to an image signal in a main scanning direction, multiple optical beam detectors that detect the deflected optical beam at two different positions on an identical main scanning line outside an image forming area, a measurement unit that measures a time period required for the optical beam to travel between the multiple optical beam detectors by counting a predetermined or given clock signal, and a determination unit that determines whether or not a normal signal is output at a timing at which the optical beam is expected to enter each of the multiple optical beam detectors.
Because it takes a relatively long time to adjust the position of the transport belt or the intermediate transfer member based on the results obtained by detecting the positioning mark, instead, the positions of the images are adjusted by adjusting the exposure timing using the optical beam detectors.
However, positions of the beam detectors can change, affected by a rise in temperature inside the optical writing unit. In such a case, accurate positional detection cannot be obtained.